1. Field of the Invention
The present invention relates to acoustics, and, in particular, to techniques for reducing room reverberation and noise in microphone systems, such as those in laptop computers, cell phones, and other mobile communication devices.
2. Description of the Related Art
Interest in simple two-element microphone arrays for speech input into personal computers has grown due to the fact that most personal computers have stereo input and output. Laptop computers have the problem of physically locating the microphone so that disk drive and keyboard entry noises are minimized. One obvious solution is to locate the microphone array at the top of the LCD display. Since the depth of the display is typically very small (laptop designers strive to minimize the thickness of the display), any directional microphone array will most likely have to be designed to operate as a broadside design, where the microphones are placed next to each other along the top of the laptop display and the main beam is oriented in a direction that is normal to the array axis (the display top, in this case).
It is well known that room reverberation and noise are typical problems when using microphones mounted on laptop or desktop computers that are not close to the talker's mouth. Unfortunately, the directional gain that can be attained by the use of only two acoustic pressure microphones is limited to first-order differential patterns, which have a maximum gain of 6 dB in diffuse noise fields. For two elements, the microphone array built from pressure microphones can attain the maximum directional gain only in an endfire arrangement. For implementation limitations, the endfire arrangement dictates microphone spacing of more than 1 cm. This spacing might not be physically desired, or one may desire to extend the spatial filtering performance of a single endfire directional microphone by using an array mounted on the display top edge of a laptop PC.
Similar to the laptop PC application is the problem of noise pickup by mobile cell phones and other portable communication devices such as communication headsets.